The metals which have historically been used to make metal eyeglass frames have usually been chosen in large part for their ease of fabrication. Metals such as nickel-silver, monel, and phosphor bronze have fairly high yield strength but quite low work-hardening, which allows them to accept large deformations during manufacture. In use, however, they tend to bend rather suddenly and in quite localized sections if their yield strength is exceeded. Such sharp bends are very difficult to removed without leaving "kinks" in the bent section. The higher strength frame materials, such as high strength stainless steels and beryllium-copper, are able to withstand much higher elastic strains without permanent deformation. They are still limited to only about 1% elastic strain, however, and if their yield strength is exceeded a bend is formed which is difficult to remove.
A number of references such as U.S. Pat. No. 4,472,035, Japanese Patent Publication No. 57-115517(A) and Japanese Patent JP-084714 have suggested the use of shape-memory alloys, especially the nickel-titanium alloys, for use as frame components due to their "superelastic" or "pseudoelastic" properties. Although these terms are often mistakenly used interchangeably, they refer to two distinctly different properties of the alloys. Careful study of all of these references, specifically U.S. Pat. No. 4,472,035 and Japanese Patent JP-084714, shows that the elastic property cited is the "pseudoelastic" property of shape-memory alloys. This pseudoelasticity occurs in a limited temperature range slightly above the stress-free austenite to martensite transformation temperature. It involves the creation of stress-induced martensite which simultaneously undergoes strain as it forms to relieve the applied stress. As soon as the applied stress is removed, the thermally unstable martensite reverts to austenite, and the strain spontaneously returns to zero. This behavior gives a very high apparent elasticity to the material without inducing any permanent strain, but is narrowly limited in the temperature range where it can be utilized in a given alloy. Because the pseudoelasticity depends upon behavior within a narrow portion of the transformation temperature spectrum, lowering the temperature as little as 10.degree. C. may change the behavior to normal shape-memory. In this case a deformed component will remain deformed unless it is heated. Also, the yield strength of this alloy, if it is annealed to give good pseudoelastic properties, is too low at low temperatures to function as a satisfactory component. Conversely, if the pseudoelastic component is heated by as little as 10.degree. C., the amount of pseudoelastic strain is significantly reduced. At even higher temperatures the pseudoelasticity is eliminated because the stress needed to stress induce the martensite exceeds the yield strength of the austenite, and permanent deformation results. Thus, the effective useful temperature range for purely pseudoelastic components may be as little as 20.degree. C. This range is too narrow to be of service to eyeglass frames which must function in winter days as cold as -20.degree. C. and in hot sunny days with possible temperatures over 40.degree. C.
Thus, while both the elastic properties and memory properties of shape-memory alloys have been discussed as potentially useful in eyeglass frames, it is clear that previous workers have not fully understood the limitations on the use of these materials, nor have they revealed any information on the proper thermo-mechanical processing necessary to utilize the alloys as frame components.